According to our personal experience, the apparatus employed by Flandin and Danger for the detection of arsenic, can also be made use of in the examination of the bisulphide of carbon solution. To this end, the fluid supposed to contain phosphorus is mixed with perfectly pure alcohol, and the mixture placed in a small spirit-lamp provided with a very loose asbestus wick. The lamp is then ignited and the flame introduced in the combustion tube D (Fig. 11).
Fig. 11.
By the combustion of the mixture, sulphurous, carbonic, phosphorous acids and water are formed. The water condenses in c, and, falling into the dish F, carries with it the sulphurous and phosphorous acids. The acid liquid collected in this way is evaporated to dryness, some nitric acid added, and the solution again evaporated. The remaining mass is then dissolved in water to which some ammonia is added, and the solution tested for phosphoric acid. This method is an advantageous one as the phosphoric acid formed must originate from phosphorus in the free state, and not from any phosphates which, owing to the presence of organic matter, might be contained in the bisulphide of carbon solution. It would, however, lead the analyst into error if the person, supposed to have been poisoned had eaten cerebral substances or eggs previous to death, as these contain glycero-phosphoric acid; it is therefore advisable to compare the results given by this process with those obtained by the use of other methods.
DETECTION OF PHOSPHOROUS ACID.
Provided free phosphorus has not been detected, it is necessary to search for phosphorous acid. To this end, the residue remaining in the flask, in either Mistcherlich's or Fresenius and Neubauer's method, is introduced into the apparatus of Dusard and Blondlot. If the phosphorus reaction appears, it is sufficient; otherwise, its production may have been hindered by the presence of organic matter. In case, therefore, the flame is colorless, the evolved gas is conducted into a neutral solution of nitrate of silver. If the materials contain phosphorous acid, a precipitate of phosphide of silver is formed which should be collected and washed. The precipitate, which is now free from organic matter, is then examined for phosphorous acid by means of the apparatus of Dusard and Blondlot.
ESTIMATION OF PHOSPHORUS.
The best process for determining quantitatively the amount of phosphorus present is the one recommended by Fresenius and Neubauer. The gaseous current is continued until a fresh nitrate of silver solution is no longer precipitated. The solution is filtered, the precipitate washed and then dissolved in nitric acid. The silver is next precipitated by addition of hydrochloric acid, the fluid again filtered, and the precipitate well washed. The washings are added to the filtrate, and the liquid concentrated in a porcelain capsule. A solution of sulphate of magnesia, containing ammonia, is next added to the fluid, and the phosphoric acid determined as pyrophosphate of magnesia: the precipitate formed, is washed, heated to redness, in order to convert it into the pyrophosphate, and then weighed.
DETECTION OF ACIDS.
The search for acids is to be instituted exclusively in the alimentary canal and its contents. Were acids contained in the other organs, their presence would be due to the blood in which they had previously been absorbed, and, as in this case they would be partially neutralized by the bases contained in the blood, a conclusive decision in regard to their original existence in the suspected materials would be impossible, the salts of the acids usually searched for being normal constituents of the blood. In order to detect the presence of acids, the alimentary canal and contents are first boiled with water which is renewed until the solution ceases to exhibit an acid reaction when tested with litmus paper. The fluid is then filtered, alcohol added to the filtrate, in order to precipitate organic substances, the liquid again filtered, and the solution tested separately for the various acids as directed below.